Vehicles of all types, whether single- or multi-track, undergo a change in the loading of the rear and front axles during braking and acceleration due to physical conditions and their structural features. The consequence is differences in grip and thus different braking actions at the rear and front wheels while braking forces remain constant. These differences in grip may cause overbraking and consequently locking due to the changing, smaller load proportion at the rear wheel during braking, even before the maximum braking force for deceleration may be reached at the front wheels and effectively utilised. This causes track instability and may result in skidding or even swerving of the vehicle in the respective phase. It is thus often no longer possible even for experienced drivers to maintain control of the vehicle, in particular of a motorcycle.
To improve this situation, pressure limiting devices, pressure regulators, load-dependent pressure regulators (e.g. DE-C-29 10 960), pressure-reducing valves, combinations of the above-mentioned components (e.g. EP-B-0 451 555) and electronically controlled ABS, ASR and vehicle dynamics control systems have been used for the rear axle in motor vehicle hydraulic brake systems.
The former components exhibit disadvantages in that, even in the case of load-dependent pressure regulators, a clear distance has to be maintained between the component-specific, stepped characteristic and the ideal characteristic brake force distribution curve, in order on the one hand to absorb the wheel brake system tolerances and on the other hand to avoid the influence of the centre of gravity height, which cannot be detected with conventional load sensing methods—see for example FIGS. 2, 11, 20, 21 and 26 in EP-B-0 451 555.
The same applies to circuits, as known for example from EP-B-0 623 079, in which, although both wheels are activated at the same time when the rear or front wheel brakes are actuated, uniform utilisation of grip is not possible for the stated reasons.
In the case of single-track vehicles already equipped with ABS, i.e. lock-protected, although an ABS modulator is associated with each axle and thus each wheel to achieve the best braking rate, which allows ABS-controlled braking on the ideal characteristic, in such systems (e.g. DE-C-35 30 280, DE-C-35 30 286, DE-C-36 03 014) the minimum achievable braking distance depends to a considerable extent, due to the separate activatability of the ABS modulators, on the instinct and skill of the driver in actuating both modulators as far as possible simultaneously. Moreover, the ideal characteristic curve is only achieved when ABS comes into play; in the case of deceleration markedly below the stability threshold, such systems have no advantages over uncontrolled brake systems.
Furthermore, the conventional behaviour of motorcyclists, which is to actuate predominantly only one brake circuit, stands in the way of achieving both the best possible braking distance and uniform utilisation of the grip at the front and rear wheels. An improvement in this regard was achieved by the circuit (semi-integrated circuit) known from the Applicant's European patent no. EP-B-0 761 520, with which the ABS modulator is activated at the rear wheel automatically when the control valve (=ABS modulator) is actuated at the front wheel. However, even in the case of this improved, driver-relieving circuit arrangement for a non-muscular-energy-assisted brake device equipped with ABS, brake status-dependent variables are not taken into account, such that optimum grip utilisation, allowing the best possible deceleration while overcoming the influence of the driver, is impossible to achieve.
The above-described disadvantages have a serious effect on driver safety, in particular in the case of motorcycles, because                in a motorcycle both the static and dynamic axle loads may vary markedly more than in the case of a car,        the ratio of admissible total weight to kerb weight is higher, and        the ratio of centre of gravity height to wheel base is plainly greater and changes more markedly than in the case of a car with its small wheel load variations and large vibration absorber action, which greatly impairs cornering behaviour of a motorcycle despite its relatively small axle mass.        
Finally, the conventional digital pressure regulation using on-off valves found in vehicle dynamics control systems is deemed a disadvantage since the load change impacts thereof generate imbalance in the running gear, in particular when used in motorcycles.
Taking as basis the generic prior art according to EP-B-0 761 520, the object of the invention is to provide a method of controlling the braking force at the wheels of a single-track vehicle, which allows the best possible automated utilisation of the grip between tyre and ground during braking, together with a simply designed brake system for implementing the method.
This object is achieved by the features indicated in claims 1 and 6 respectively. Advantageous or expedient further developments of the invention constitute the subject matter of claims 2 to 5 and 7 to 12.